Method and device for moving subsea rocks and sediments

ABSTRACT

A method for moving subsea rocks and sediments which can be at significant depths, in connection with removal of protective rocks around installations where maintenance is to be conducted. Tubing with an ejector nozzle is arranged completely externally in relation to the tubing on a standard, remote controlled submarine (ROV), so that the ejector nozzle produces a pressure gradient through the tubing. An inlet end of the tubing is located with the rocks and sediments to be moved, which are sucked into the tubing and blown out of the opposite end of the tubing.

BACKGROUND OF THE INVENTION

For work at subsea oil and gas installations or in connection with suchinstallations, e.g. maintenance work, there is often a need to moverocks and particulate material that partly covers the body that is to berepaired. The body can be a pipeline, valve housing and the like.

In a similar way it may arise a need to remove sediments in connectionwith new installations on the sea bottom, or for removal of accumulateddrill cuttings at platforms or the like.

Similar needs may also occur in connection with subsea work, such as atharbour works or barrage or quay structures.

DESCRIPTION OF RELATED ART

The most common way to remove sediments in connection with subsea work,is to utilize large “fans”, large and heavy suction devices with a highpower consumption and specially designed excavators. Disadvantages arethat they require a lot of power and/or other resources, they requirelarge surface vessels, have a limited versatility, are as good asstationary, or they are not at all suited for deep waters.

NO patent No. 302.043 describes a dredge designed for subsea operations,especially to remove or move drill cuttings. The dredge comprises amotor, a pump device and an ejector. The motor is designed to run thepump which in its turn provides a stream of water to the ejector. Theejector is positioned in a tubing through which the cuttings or the likeare supposed to be transported. The apparatus is designed to rest on thesea bottom and to receive energy from the surface, while the inlet endof the tubing is intended to be moveable e.g. with the aid of a remotecontrolled mini submarine, a so called ROV.

This apparatus is not suited to move sediments with relatively largerocks, mainly because the pipeline has an effective loss of diameter dueto the ejector's design and position. Furthermore it has geographicallya very limited work range, as it is designed to rest at the sea bottom,even though the pipeline is supposed to be somewhat moveable.

Japanese patent applications Nos. 043 25 799 A and 043 25 800 Adescribes an ejector pump system where the ejector is positioned mainlyoutside the pipeline so that the ejector does not reduce the effectivediameter of the pipeline. From the abstract of these patent applicationsit is not possible to see what kind of utilizations these systems areintended for. Neither are there any indications of dimensions or powerrequirements for these systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method fortransportation of rocks and sediments under water, especially in deepwaters.

It is a particular object to provide a method for transportation ofrocks with a typical maximum diameter of about 200 mm.

It is further an object to provide an apparatus for performing saidmethod, which apparatus should be versatile in its use, especially inthe way that it should be easy to move around down at the sea bottom.

It is still further an object to provide such an apparatus which is easyto control, and which does not require large amounts of energy to besupplied from the surface, but which can utilize the standard powersupply available on an ROV.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following an example of an apparatus according to the inventionis described in further detail with reference to the accompanyingdrawing.

FIG. 1 is a schematic view of a preferred embodiment of an apparatusaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows in a very simplified form a remote controlled submarine(ROV) 1 with a suspension member 2 and a power cord 3. The suspensionmember 2 and the power cord 3 both extend up to the sea surface (notshown). The ROV 1 is also provided with a number of propellers 4 toenable it to move, one of which is shown in the drawing.

Arranged on the ROV 1 is a tubing 5 with an inlet 6 and an outlet 7. Thetubing 5 may have at least one flexible section 8, permitting the inlet6 of the tubing to be moved by means of a manipulator 9 that can becontrolled independently of the ROV. The edge 10 of the inlet 6 isrounded to minimize loss at the inlet. The outlet 7 is shaped as adiffusor, also to minimize loss and thereby the power requirement. Thetubing 5 is preferably designed with a substantially even cross sectionfrom its inlet to its outlet and without any sharp bends where largerocks might be trapped.

An ejector nozzle 11 is attached externally to the tubing 5 so that itdoes not diminish the diameter of the tubing. The ejector nozzle 11provides a powerful water jet supplied from a water pump 12 powered by ahydraulic member 13. The ejector nozzle 11, the water pump 12 and thehydraulic member 13 together constitute the power assembly or the“motor” for the apparatus according to the invention.

Close to the inlet 6 of the tubing a “backflushing” arrangement maypreferably be arranged to blow out rocks which are too large or cloggedsediment that might get stuck in the inlet. Further an opening 15 may beprovided through the inlet 6 or through the tubing 5 close to the inlet,the size of which is preferably adjustable for the purpose ofcontrolling the suction at the inlet point and reducing its tendency toget stuck in fine sediments.

The apparatus according to the invention may be primarily manufacturedfrom plastic material with a specific weight close to that of water, sothat it is easy for an ROV to carry.

All the mentioned units are fixed or moveable on the ROV 1 and the powerconsuming components all receive power through the ROV's standard powersupply 3.

The drawing also indicates a collection of rocks 14 in the process ofbeing moved through the tubing 5 by means of the described mechanism.

Calculation Example

In the following calculation example it is presupposed that theapparatus is attached to a ROV with an available 30 kW (hydraulic)effect. It is further presupposed that the hydraulic motor and the waterpump have a total efficiency of 0.64. In that case the water pump willbe able to provide 3000 l/min. at a pressure of 4.0 bar. Rocks used toprotect structures at the sea bottom usually have a largest diameterd_(max)=150 mm. If the tubing has a diameter of 250 mm and a length of8.0 m, the following theoretical data follows:

TABLE 1 Velocity before mixing chamber 5.0 m/s Velocity after mixingchamber 6.0 m/s Required velocity 4.0 m/s Motive power (lifting height)1.50 m of which inlet loss 0.26 m (k = 0.2) frictional loss 0.68 m (k =0.017) outlet loss 0.56 m (k = 0.3) Capacity transport of 150 mm rocks20 t/hour (confirmed by model tests) Power consumption ROV 30 kW Powerfrom water pump 19 kW Net effect used by the device  4 kW

Example of Practical Embodiment

In a commission during the summer of 1999 1500 m³ of rocks (d_(max)=150m) were moved with an apparatus according to the present invention. Thecommission took place in Tengsfjorden, at a pipeline for oil at a 540 mdepth. An ROV of type Perry-XL was used. 100 l/min. of hydraulic oil ata pressure of 180 bar was available for running the water pump, whichcorresponds to 29 kW. The tubing, made from PE-plastic, had a totallength of 10 m, and an internal diameter of 250 mm. Only a minimal wearcould be observed on the tubing, It was observed, however, that rocksfrom time to time got stuck due to unintended unevenness in the tubing.The invention is unique in the sense that it allows rocks to be movedwith much lighter equipment than that previously available on themarket, and at depths where it has not been possible to conduct thiskind of task. The above-referred commission confirms that the inventionis practically feasible.

1. Method for moving subsea rocks and sediments, comprising disposing,in combination with a remote controlled submarine (ROV) havingpropulsion means, a tubing having a substantially constant cross sectionand a first end adjacent the ROV and a second end adjacent the ROV, acontrollable aperture to control suction force in said first end, awater pump, an ejector nozzle connected to the water pump for expellinga stream of water in a direction toward the second end of the tubing,the ejector nozzle being disposed externally to the tubing, moving thefirst end of the tubing to contact the subsea rocks and sediments to bemoved, and establishing a controlled pressure gradient through thetubing by moved expelling the stream of water toward the second end ofthe tubing, whereby the rocks and sediment are expelled through thesecond end of the tubing.
 2. Method according to claim 1, wherein waterpump is supplied with energy by the energy supply of said ROV.
 3. Devicefor moving subsea rocks and sediments, comprising a remote controlledsubmarine (ROV) including propulsion means, and including in combinationtherewith, tubing having a substantially constant cross section and afirst end constructed and arranged for contacting the rocks and sedimentto be moved and a second end constructed and arranged for expelling therocks and sediment to be moved away from the ROV, a controllableaperture for controlling suction force in said first end, and means forcreating a pressure gradient in the tubing comprising a water pumpconnected to an ejector nozzle for expelling water toward the second endof the tubing, the ejector nozzle being disposed externally to thetubing.
 4. Device according to claim 3, wherein all required energy issupplied through at least one cable from the surface to the ROV. 5.Device according to claim 3, wherein the first end of the tubing ismovable by means attachable to the ROV.
 6. Device according to claim 3,wherein an inlet mouth piece attached to the first end of the tubingcomprises a rounded edge in order to reduce inlet loss.
 7. Deviceaccording to claim 3, wherein the second end of the tubing comprises anoutlet shaped as a diffusor in order to reduce outlet loss.
 8. Deviceaccording to claim 3, comprising parts thereof made from plasticmaterials with a density close to the density of water, to enable easysupport by an ROV.